1. Field of the Invention
The present invention relates to a DC motor, and more particularly to a cored DC motor with armature coils wound on an iron core provided on the rotary shaft and capable of providing a high torque at a low-speed rotation.
2. Description of the Prior Art
In recent years, DC motors for driving electronic appliances are increasing used in so-called direct driving system in which the motor is directly connected to an object member without intermediate gears or belts. For this reason, there has been required a smaller and lighter motor capable of achieving a high torque and a low-speed rotation with uniform torque and rotation speed.
In order to achieve a lower rotation speed and a higher torque in a motor of a given form and dimension, there is already known the use of an increased number of poles in the field system. More specifically, the revolution N decreases while the torque T increases in accordance with an increase in the number p of poles as represented by following general relations for a motor: ##EQU1## wherein: V: voltage applied
I: current PA1 R: resistance between terminals PA1 Z: total number of effective conductors PA1 .PHI.: number of effective magnetic fluxes PA1 a: number of parallel circuits PA1 p: number of poles. PA1 K: torque constant (g.cm/A) PA1 R: resistance between terminals (.OMEGA.). PA1 Ts: starting moment (g.cm) PA1 N.sub.0 : revolution without load (rpm).
The use of multiple poles in small DC motors is common mainly in brushless motors, such as shown in FIG. 1 which corresponds to that disclosed in Japanese Utility Model Application Laid-open No. 52614/1977, but in commutator motors, the use of such multiple poles is limited to the use of four poles in 3-slot motors. In such 3-slot motors the winding density remains same both for 2 poles and for 4 poles as will be explained later, and the use of multiple poles leads to a decrease in the number of effective fluxes .PHI. as long as the material of magnets is not changed. For this reason, a significant improvement in the motor performance cannot be expected in such motors.
Although a lower rotation speed and a higher torque are already achieved in brushless motors through the use of multiple poles, the brushless motors themselves are becoming unsuitable for certain appliances such as video tape recorders in consideration of the recent trend toward a smaller and lighter mechanism and toward a lower price.
The motor of the present invention can be manufactured with a cost comparable to that of the conventional cored motors and still is capable of providing a marked improvement in achieving a lower rotating speed, a higher torque and a smaller and lighter structure.
As already known, in order to expand the torque increase rate (m) for a given decrease in the motor revolution, it is necessary to increase a value equal to the square of torque constant divided by the resistance between terminals: EQU m=1.027.times.(K.sup.2 /R).times.10.sup.-5
wherein:
Said value m represents the increase in torque per decrease in revolution and is expressed by m=.DELTA.T/.DELTA.N.apprxeq.Ts/N.sub.0
wherein:
As explained before, said value m is proportional to K.sup.2 while K.varies.Z.PHI., so that there is obtained a relation m.varies.Z.sup.2 /R since .PHI. can be considered approximately constant for a given material of magnet and a given dimension.
As long as a given structure is assumed for the motor, the value of R increases with an increase in Z but the value of m remains substantially constant.
Now let us consider the effect of the number p of poles as a factor influencing on the values of N and T.
For an armature core of a given shape, a coil structure for two poles involves, as shown in FIG. 2, a long coil pitch L.sub.1 and mutually overlapping coils l.sub.1 -l.sub.5, thus giving rise to long end connections and to a large axial dimension as shown in FIG. 3.